Laser diode drive method and arrangement

ABSTRACT

A method and apparatus to drive a laser diode are disclosed comprising increasing a bias current to the laser diode to a threshold level, wherein the threshold level is below an actuation level of the laser diode and wherein a resistor is placed in parallel to the laser diode, charging a capacitance to a precharge capacitance of a circuit including the laser diode, wherein the precharge capacitance is below a capacitance actuation level of the laser diode; and actuating the laser diode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 62/714,027, filed Aug. 2, 2018, which is herein incorporated byreference.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

Embodiments of the present disclosure generally relate to laser diodes.More specifically, embodiments of the present disclosure relate to laserdiode drive methods and arrangements.

Description of the Related Art

A Laser Phosphor Display (LPD) generates video images by illuminatingpixels using multiple focused laser beams that are scanned across thescreen. Each pixel embedded in the display screen contains phosphormaterial that radiates light proportional to the laser beams power andtime over the pixel. The brightness of each pixel can therefore becontrolled by a combination of the laser diodes peak drive current andpulse width duration. A high quality LPD display needs to achieve highresolution (pixels are close together) and a wide optical dynamic rangein brightness levels. When multiple lasers are used, it is also criticalthat the drive circuits be low cost and simple in construction.

Achieving perfect blacks is very critical to a Seamless LPD displaybecause of the overlap of each light engines image region. The blacklevels add in the overlap regions resulting in a checkerboard visualaffect that the viewer would see unless the blacks are truly black. Thepresent drive method requires a trade-off between perfect blacks andpulse performance due to the simplicity and low cost of the architectureand the non-linear nature of a laser diode.

The conventional methods and apparatus have several limitations. Thefirst limitation of the present design is that the bandwidth of thedriver reduces as the peak current level is reduced. The lower bandwidthresults in slower rise and fall times and ultimately makes it verydifficult to achieve optical power control in what is called ‘the lowgrey region’ of the display curve. One method to improve the linearityin ‘the low grey region’ involves raising the laser diode dc biascurrent. Unfortunately, since the LD operates like an LED at lowcurrents, the screen phosphors still receive sufficient illuminationresulting in black levels looking grey.

A second limitation of the existing drive method is that because thepixels are close together, the peak level of the driver pulse may beaffected by the previous pulse or pulses. This makes it more difficultto control the linearity of the image and can even result in a pixelfailing to illuminate if it was preceded by a black region of sufficientduration. Both limitations can be minimized using more advancedprocessing in software and FPGA hardware, but it is very desirable toeliminate this interaction.

The third limitation arises from the non-linear nature of a laser diode.This presents a non-linear load for the driver circuit that makes ithard to optimize over the entire operating range.

A fourth limitation is the calibration required to control the previouslimitations. Optical factory equipment must be employed to measure blacklevels and balance the driver currents since all laser diodes havedifferent output characteristics vs current.

There is a need to provide a method to drive a laser diode that issuperior to the conventional driving methods.

There is a need to provide a method wherein the bandwidth of the driverdoes not reduce as the peak current level is reduced.

There is further need to provide a method wherein a peak level of thedriver pulse is not affected by a previous pulse or pulses.

There is a further need to provide a method that loads for a drivercircuit are optimized over an entire operating range.

SUMMARY OF THE DISCLOSURE

In one embodiment, a method to drive a laser diode is disclosedcomprising increasing a bias current to the laser diode to a thresholdlevel, wherein the threshold level is below an actuation level of thelaser diode and wherein a resistor is placed in parallel to the laserdiode, charging a capacitance to a precharge capacitance of a circuitincluding the laser diode, wherein the precharge capacitance is below acapacitance actuation level of the laser diode, and actuating the laserdiode.

In another embodiment, a method to drive a laser diode is disclosedcomprising increasing a bias current to the laser diode in a series ofpulses to a threshold level, wherein the threshold level is below anactuation level of the laser diode and wherein a resistor is placed inparallel to the laser diode and wherein the series of pulses are greaterin frequency than a laser diode current discharge rate, charging acapacitance to a precharge capacitance of a circuit including the laserdiode, wherein the precharge capacitance is below a capacitanceactuation level of the laser diode and actuating the laser diode.

In another embodiment, an arrangement for providing a current to anapparatus, is disclosed comprising a diode, a resistor placed inparallel to the laser diode, at least two transistors, wherein eachtransistor has a collector, an emitter and a base, and each collector isconnected to the laser diode, at least one operational amplifierconnected to each base of the at least two transistors, a direct currentpower supply connected to the at least one operational amplifier and atleast one direct current power supply connected to each of the emittersof the at least two transistors.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentdisclosure can be understood in detail, a more particular description ofthe disclosure, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this disclosure and are therefore not to beconsidered limiting of its scope, for the disclosure may admit to otherequally effective embodiments.

FIG. 1 is a curve of an optical output power vs forward current for alaser diode.

FIG. 2 is typical modulation for a curve of an optical output power vsforward current for a laser diode.

FIG. 3 is an conventional arrangement for controlling laser diodecurrent.

FIG. 4 is graph of laser diode resistance vs current.

FIG. 5 is a graph of driver transistor beta vs collector current.

FIG. 6 a graph of drive open loop gain vs laser current.

FIG. 7 is a graph of a conventional driver method with no bias resistorfor a laser diode.

FIG. 8 is a graph of an improved driver method in one embodiment of thedisclosure.

FIG. 9 is a graph of a conventional driver method with no bias resistorfor a laser diode.

FIG. 10 is a graph of a conventional driver method with no bias resistorfor a laser diode.

FIG. 11 is a graph of an arrangement for driving a laser diode current.

FIG. 12 is a graph of driver load resistance, Rbias and current vsdriver current.

FIG. 13 is a graph of voltage over time with no precharge for a laserdiode.

FIG. 14 is a graph of voltage over time for a single low precharge for alaser diode.

FIG. 15 is a graph of voltage over time for two pre-charge pulses for alaser diode.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe FIGs. It is contemplated that elements disclosed in one embodimentmay be beneficially utilized on other embodiments without specificrecitation.

DETAILED DESCRIPTION

In the following, reference is made to embodiments of the disclosure.However, it should be understood that the disclosure is not limited tospecific described embodiments. Instead, any combination of thefollowing features and elements, whether related to differentembodiments or not, is contemplated to implement and practice thedisclosure. Furthermore, although embodiments of the disclosure mayachieve advantages over other possible solutions and/or over the priorart, whether or not a particular advantage is achieved by a givenembodiment is not limiting of the disclosure. Thus, the followingaspects, features, embodiments and advantages are merely illustrativeand are not considered elements or limitations of the appended claimsexcept where explicitly recited in a claim(s). Likewise, reference to“the disclosure” shall not be construed as a generalization of anyinventive subject matter disclosed herein and shall not be considered tobe an element or limitation of the appended claims except whereexplicitly recited in a claim(s).

In one embodiment, a driver method for a laser diode is disclosed. Thedriver method provides a way to achieve the full potential of LDPtechnology by allowing laser diode ‘off’ current to reach zero whilestill maintaining high bandwidth pulse control of the laser diodes peakcurrent waveform. The method also reduces the ‘history effect’ bypreventing the pulse current from one pixel from affecting other pixels.The driver method also simplifies calibration of the product byeliminating optical factory equipment to fine tune the bias current foreach laser channel (typically 20 channels) compared to conventionalmethods. In embodiments, a board tester is used to set all channels to apre-determined bias value.

Background on Laser Diode Driving

FIG. 1 shows a laser diode curve of output power in mw vs current in mafor a conventional application for driving a laser diode. There are twodistinct regions of operation, below threshold (LED region) and abovethreshold (laser region). In all laser diode applications, the goal isto operate the device above threshold in the lasing region where opticaloutput rises linearly with increasing current.

In typical pulsed applications, a small residual ‘OFF’ LED output levelmay be tolerated so circuits are not required to dynamically reduce thelaser diode current to zero. Instead, in conventional applications, mostlaser diode arrangements operate with a dc bias ‘Off’ level shown inFIG. 2 either just below the knee or just above the knee to keep thedriver in a linear operating range and support a fast entry into thelasing region.

In an LDP display application, however, it is desirable to set the ‘OFF’dc level as close to zero as possible to minimize any LED opticaloutput. In an LDP display, systems are operated in the lasing regionabove threshold and must achieve a wide brightness range from severalhundred nits down to a fraction of a nit. The bright end of the range isachieved by generating fast rise time 1000 mw optical pulses thatapproach a 50% on to off duty cycle to fully illuminate each phosphorpixel region. The low end of the range is achieved by generating currentpulses that are very narrow and with amplitudes that exceed the laserthreshold. The low end of the dynamic range, described above, requiresthat the ‘off’ or black level is less than 1 mnit, which can only beachieved with less than 1 ma of laser diode current. Since every laserdiode will output a different optical output level for the same drivecurrent, the circuits must be very precise or require carefulcalibration to achieve good background uniformity across the screen.

Conventional Methods to Control Laser Diode Current

FIG. 3 illustrates a typical drive circuit for a conventional way ofcontrolling the laser diode current through the laser diode identifiedas LD1. The circuit consists of op amp U1 driving the base of two highspeed transistors Q1 and Q2 wired in parallel. Two transistors are usedto provide up to one amp peak current. The anode of LD1 is connected toLSR_PWR that, in one embodiment, is set to +8 volts. The cathode of LD1is connected to the collectors of Q1 and Q2. The two emitters areconnected to current sense resistor R1 that converts the current flowingthrough the transistors to a voltage labeled Vsense that is connectedthrough feedback resistor R2 to the negative input of U1. This completesa feedback loop that converts the Pulse_In voltage into a current pulsethat flows from the LSR_PWR voltage source through the laser diode LD1and into Q1 and Q2. The input signal labeled Pulse_In is typically theoutput from an analog switch whose input is an adjustable voltagesource. When the switch is turned on, the voltage source is connected toPulse_In and results in a laser diode current pulse until the switch isclosed. A second input to the circuit is an adjustable voltage sourcelabeled Vbias that is set to correct for any offset errors in theamplifier U1 and minimize the LD current when Pulse_In is off. Note thatPulse_In typically consists of voltage pulses with rise and fall timesless than 5 ns and pulse on times ranging from 5 to 100 ns and can beconsidered near ideal.

There are several limitations of this circuit that affect theperformance when attempting to minimize laser diode off current andmaximize pulse performance. These limitations are primarily due to thecircuits low cost and simple architecture. Laser diodes are verysensitive to momentary negative voltages and over currents so thisdesign only uses positive supplies to provide reliable operation. Thiscircuit uses an open collector topology and controls the laser currentby sinking current from the laser into the collectors under closed loopcontrol. The circuit can therefore increase the laser current veryquickly by driving more base current into the transistors, but relies onthe laser impedance to reduce the current. Also, when the laser ispositioned farther away from the driver, there will be additionalcapacitance from the collector to ground and from the collector toLSR_PWR that will slow down the turn off.

Turn off is complicated by the fact that a laser diodes voltage drop andresistance varies non-linearly with current as shown in FIG. 4. As thecurrent increases above threshold (about 120 ma), the resistanceapproaches a few ohms, but at 1 ma or less, the resistance can easilyreach 10K or higher. The laser diode also has a forward voltage drop ofseveral volts.

When high peak pulse currents are applied to the laser diode, the turnoff times are very fast since the laser's low resistance provides a fastdischarge path. When this non-linear device is driven with the circuitof FIG. 3, the pulse response will begin to degrade as we lower the biasor ‘OFF’ current close to 0 ma. The increase in laser diode resistancewith lower current results in a slower decay of laser diode current.This makes the circuit sensitive to increased capacitance which limitslocating the laser diode away from the driver.

A second limitation of this circuit is due to the non-ideal nature ofthe transistors Q1 and Q2. These transistors are chosen for their highfrequency and high current drive characteristics and consequently theirgain or beta drops at lower currents. Referring to FIG. 5, a plot oftransistor Beta=IC/IB vs current for the typical parts used in theexisting driver is shown. As the current falls from about 15 ma to 0.8ma (first point), it is seen that the combined transistor gain drops byabout 50%. Since the circuit uses closed loop control to preciselycontrol the current level, the close loop control results in a reductionin driver bandwidth as the current is reduced.

One way to measure the limitations of the conventional method is to plotthe open loop gain in the feedback loop vs dc bias current. From controltheory, the bandwidth of the driver is proportional to the frequency inHz where the open loop gain equals unity. This is plotted in FIG. 6 forthe existing and new driver method. As seen in the figures, the openloop gain of both methods is the same for laser currents above 150 ma,but the existing method drops down to 1 Mhz at 1 ma. The new methodraises this to 17 Mhz which is sufficient to produce very good controlof the laser drive currents. A second way to show the limitations of theexisting method and improvements in the new drive method is to measureand compare the optical pulse response.

FIGS. 7, 8, 9 and 10 show the improvement of the described embodimentsover conventional drive methods when operating in the most difficultregion of narrow pulse widths and low amplitude pulses. The opticalpulse response of the laser diode is measured by focusing the laser'soutput onto a high speed optical detector. For each of these FIGs, aburst of 5 identical pulses is generated and the pulse width is adjustedfor identical steady state peak detector outputs. FIGS. 7 and 8 comparethe existing (conventional) and new method and apparatus for 100 mv peakpulses. As seen in FIGS. 7 and 8, the first pulse amplitude in the burstis about 50% lower than the following pulses for the existing method.FIG. 8 shows that all 5 pulses are close to the same amplitude for theaspect described. A second observation is that the pulse width for theconventional method is 9.7 ns and has been reduced to 9.2 ns for the newaspects described. Both improvements are consistent with the bandwidthincrease shown in FIG. 6. FIGS. 9 and 10 compare the existing(conventional) and new method for 50 mv peak pulses. In this case, theconventional method fails to generate the first pulse but, unlike in inthe conventional diagram, all five (5) pulses are present for the newmethod. Note both methods will result in all five pulses reaching thesame amplitude as the pulse width is increased and the peak levelincreases.

FIG. 11 illustrates one aspect of the disclosure of a drive methodconsisting of the addition of a single 200 ohm resistor placed inparallel with the laser diode. This resistor is called a ‘Rbias’resistor. The Rbias resistor works in conjunction with Vbias and thelaser diode non-linear transfer curve to provide a substantialimprovement and cost-effective method to control the pulsed laser diodecurrent. The curves of FIG. 4 show that the voltage drop across thelaser diode has a sharp knee close to 3V that occurs at a low currentlevel. We also know that below about 15 ma, the gain of transistors Q1and Q2 will start to reduce contributing to a loss of open loop gain.Adding the proper resistor value in parallel with the laser diode incombination with setting the DC bias current through the resistor willallow the driver to operate at a minimum current and simultaneouslyallow the laser diode current to approach zero. In the case of driving alaser diode, setting the bias current to 15 ma will yield a 3 V dropacross Rbias and result in a laser diode current of less than 0.1 ma. Itis very desirable to set this voltage to match the laser diode forwardthreshold value since this minimizes the voltage swing from ‘OFF’ to‘ON’ required to drive the laser resulting in faster optical rise andfall times.

The above is further shown in FIG. 12. In this plot, the driver currentswept from 0 to 36 ma on the horizontal axis and the current is plottedthrough the bias resistor, the current through the laser diode, and thecombined parallel resistance of the bias resistor and the laser diode.This plot shows the bias current may be increased in the driver up to 15ma and still result in the laser diode current remaining at zero. As thedriver current is increased, the laser diode resistance drops resultingin most of the driver current flowing into the laser diode. This becomeseven more effective as the laser diode is driven with higher currents.As expected, the combined driver load resistance=200 ohms at zerocurrent and then transactions to several ohms as the current isincreased.

The 200 Ohm resistor plays a significant role in reducing the effects ofcapacitance between the collectors and ground and also between thecollectors and the LSR_PWR source. When the driver current is switchedoff the current decay is dominated by the RC time constant nowcontrolled by the resistor at lower currents. This results in asignificant improvement in preventing one pulse from affecting the nextpulse as the time between pulses becomes short.

Lastly, the introduction of Rbias provides a method to calibrate the‘OFF’ optical output level. Instead of relying on optical measuringequipment as required in conventional methods, every circuit iscalibrated by simply opening the switch that applies Pulse_IN to thedriver and adjusting the Vbias voltage to produce three (3) volts acrossthe laser diode terminals. This works because at this voltage all laserdiodes will result in very low currents due to their high offresistance. This calibration can be performed with or without the laserdiode connected allowing each circuit to be calibrated when the pcb istested.

Pre-Charge Method

A pulse command method called ‘Pre-Charge’ is now described in anon-limiting embodiment, that provides additional margin to guaranteethat the first pulse in a burst will always occur and prevent a missingpixel on the display screen. As provided in FIG. 9, it is possible forthe first pulse in a burst that occurs after a sustained off period tofail to illuminate a pixel. This potential failure mechanism, however,is greatly improved with the drive method described herein. It isimportant to note, however, that when a laser diode deviatessubstantially from a normal condition, a failure to light may stilloccur. To handle these cases, a ‘Pre-Charge’ method consisting ofcommanding a single low amplitude pulse or a burst of normal amplitudepulses can be used to further increase the bias current and charge anycapacitance in the laser diode electrical path.

As provided in FIG. 3, a ‘Pulse In’ is typically from an adjustablevoltage source connected through an analog switch. FIG. 13 shows anexample case where pre-charge can improve what is defined as the ‘firstpixel up’ after a long period of no pulses. In FIG. 13 the trace 1300 isthe ‘Pulse In’ signal input to the driver. The trace 1302 shows thelaser diode current pulses for the first two pixels in a burst. The peakcurrent of the first pulse is slightly lower than the second pulse dueto capacitance charging. The resulting laser output power is illustratedin 1304 and it is seen that very good current control is required toresult in consistent peak output power.

FIG. 14 shows a graph that uses a ‘pre-charge’ of the laser andelectrical connection capacitance to produce consistent output opticalpower levels. In this case, a low amplitude pulse of voltage is appliedjust prior to the larger pulse used to illuminate the pixel. The lowamplitude pulse is typically adjusted to result in a low current pulsethat is well below threshold (about 5 to 10 ma) just prior to the highamplitude pulse used to illuminate the pixel. In FIG. 14, the lowpre-charge pulse results in the first and second pulses matching inoptical output.

A second alternative method for a cost-effective design is to takeadvantage of the fact that the laser diode current rise and fall timesare slower than the ‘Pulse In’ waveforms. In this case ‘Pre-charge’ isperformed using a pulse control method that switches the ‘Pulse In’signal on and off quickly to let the driver low pass filter the pulsetrain input. By properly modulating the On-Off pre-charge switch timing,we can achieve an average bias current level that matches the firstmethod in performance. This is shown in FIG. 14. The benefits of theembodiment is that it can be implemented using the existing circuitsthat are already in place.

In one non-limiting embodiment, a method to drive a laser diode, isdisclosed comprising increasing a bias current to the laser diode to athreshold level, wherein the threshold level is below an actuation levelof the laser diode and wherein a resistor is placed in parallel to thelaser diode, charging a capacitance to a precharge capacitance of acircuit including the laser diode, wherein the precharge capacitance isbelow a capacitance actuation level of the laser diode and actuating thelaser diode.

In another non-limiting embodiment, the method may be performed whereinthe increasing the bias current to the laser diode is performed byproviding a single amplitude pulse of current to the laser diode.

In another non-limiting embodiment, the method may be performed whereinthe increasing the bias current to the laser diode is performed byproviding a burst of amplitude pulses to the laser pulses.

In another non-limiting embodiment, the method may be performed whereinincreasing the bias current to the laser diode is through an adjustablevoltage source.

In another non-limiting embodiment, the method may be performed whereinthe adjustable voltage source is controlled through an analog switch.

In another non-limiting embodiment, the method may be performed whereinthe increasing the bias current to the laser diode to the thresholdlevel is through sweeping a current from a zero level to the thresholdlevel.

In another non-limiting embodiment, a method to drive a laser diode isdisclosed comprising increasing a bias current to the laser diode in aseries of pulses to a threshold level, wherein the threshold level isbelow an actuation level of the laser diode and wherein a resistor isplaced in parallel to the laser diode and wherein the series of pulsesare greater in frequency than a laser diode current discharge rate;charging a capacitance to a precharge capacitance of a circuit includingthe laser diode, wherein the precharge capacitance is below acapacitance actuation level of the laser diode and actuating the laserdiode.

In a further non-limiting embodiment, the method may be performedwherein the increasing the bias current to the laser diode is through anadjustable voltage source.

In a further non-limiting embodiment, the method may be performedwherein the adjustable voltage source is controlled through an analogswitch.

In a further non-limiting embodiment, the method may be performedwherein the increasing the bias current to the laser diode to thethreshold level is through sweeping a current from a zero level to thethreshold level.

In a further non-limiting embodiment, an arrangement for providing acurrent to an apparatus is disclosed comprising a diode, a resistorplaced in parallel to the laser diode, at least two transistors, whereineach transistor has a collector, an emitter and a base, and eachcollector is connected to the laser diode, at least one operationalamplifier connected to each base of the at least two transistors, adirect current power supply connected to the at least one operationalamplifier, and at least one direct current power supply connected toeach of the emitters of the at least two transistors.

In a further non-limiting embodiment, the arrangement may furthercomprise at least one capacitor placed in parallel to the at least oneoperational amplifier.

In a further non-limiting embodiment, the arrangement may be performedwherein the at least two transistors is a first transistor and a secondtransistor.

In a further non-limiting embodiment, the arrangement may furthercomprise at least two resistors positioned in between the emitters ofthe first transistor and the second transistor and the at least onedirect current power supply connected to each of the emitters.

In a further non-limiting embodiment, the arrangement may be configuredwherein the at least two resistors is a first resistor and a secondresistor.

In a further non-limiting embodiment, the arrangement may be configuredwherein the first resistor has a higher resistive value than the secondresistor.

In a further non-limiting embodiment, the arrangement may furthercomprise at least one capacitor placed in parallel with one of the atleast two resistors positioned in between the emitters of the firsttransistor and the second transistor.

In a further non-limiting embodiment, the arrangement may furthercomprise at least one resistor placed between the emitters of the firsttransistor and the second transistor and ground.

In a further non-limiting embodiment, the arrangement may be configuredwherein the diode is a laser diode.

While the foregoing is directed to embodiments of the presentdisclosure, other and further embodiments of the disclosure may bedevised without departing from the basic scope thereof, and the scopethereof is determined by the claims that follow.

What is claimed is:
 1. A method to drive a laser diode, comprising:increasing a bias current to the laser diode to a threshold level,wherein the threshold level is below an actuation level of the laserdiode and wherein a resistor is placed in parallel to the laser diode;charging a capacitance to a precharge capacitance of a circuit includingthe laser diode, wherein the precharge capacitance is below acapacitance actuation level of the laser diode; and actuating the laserdiode.
 2. The method according to claim 1, wherein the increasing thebias current to the laser diode is performed by providing a singleamplitude pulse of current to the laser diode.
 3. The method accordingto claim 1, wherein the increasing the bias current to the laser diodeis performed by providing a burst of amplitude pulses to laser pulses.4. The method according to claim 1, wherein increasing the bias currentto the laser diode is through an adjustable voltage source.
 5. Themethod according to claim 4, wherein the adjustable voltage source iscontrolled through an analog switch.
 6. The method according to claim 1,wherein the increasing the bias current to the laser diode to thethreshold level is through sweeping a current from a zero level to thethreshold level.
 7. A method to drive a laser diode, comprising:increasing a bias current to the laser diode in a series of pulses to athreshold level, wherein the threshold level is below an actuation levelof the laser diode and wherein a resistor is placed in parallel to thelaser diode and wherein the series of pulses are greater in frequencythan a laser diode current discharge rate; charging a capacitance to aprecharge capacitance of a circuit including the laser diode, whereinthe precharge capacitance is below a capacitance actuation level of thelaser diode; and actuating the laser diode.
 8. The method according toclaim 7, wherein increasing the bias current to the laser diode isthrough an adjustable voltage source.
 9. The method according to claim8, wherein the adjustable voltage source is controlled through an analogswitch.
 10. The method according to claim 7, wherein the increasing thebias current to the laser diode to the threshold level is throughsweeping a current from a zero level to the threshold level.
 11. Anarrangement for providing a current to an apparatus, comprising: adiode; a resistor placed in parallel to the diode; at least twotransistors, wherein each transistor has a collector, an emitter and abase, and each collector is connected to the diode; at least oneoperational amplifier connected to each base of the at least twotransistors; a direct current power supply connected to the at least oneoperational amplifier; and at least one direct current power supplyconnected to each of the emitters of the at least two transistors. 12.The arrangement according to claim 11, further comprising: at least onecapacitor placed in parallel to the at least one operational amplifier.13. The arrangement according to claim 11, wherein the at least twotransistors is a first transistor and a second transistor.
 14. Thearrangement according to claim 13, further comprising: at least tworesistors positioned in between the emitters of the first transistor andthe second transistor and the at least one direct current power supplyconnected to each of the emitters.
 15. The arrangement according toclaim 14, wherein the at least two resistors is a first resistor and asecond resistor.
 16. The arrangement according to claim 15, wherein thefirst resistor has a higher resistive value than the second resistor.17. The arrangement according to claim 14, further comprising: at leastone resistor placed between the emitters of the first transistor and thesecond transistor and ground.
 18. The arrangement according to claim 17,wherein the diode is a laser diode.
 19. The arrangement according toclaim 13, further comprising: at least one capacitor placed in parallelwith one of the at least two transistors positioned in between theemitters of the first transistor and the second transistor.
 20. Thearrangement according to claim 11, wherein the diode is a laser diode.